Cavity resonator



United States Patent() 3,278,863 CAVITY RESONATR Alfred Kch, Nussbaumen, Aargau, Switzerland, assigner to Patelhold Patentverwertungs- & Elektro-Holding AG., Glarus, Switzerland Filed Dec. 10, 1964, Ser. No. 417,367 Claims priority, application Switzerland, Dec. 17, 1963, 15,435/63 16 Claims. (Cl. S33-83) The present invention relates to cavity resonators yfor use in connection with the transmission of microwaves, and more particularly, though not limitatively, to an improved coupling or impedance matching arrangement for the feeding to and extraction of the microwave energy from a cavity resonator serving as an intermediate transmission or circuit element of a microwave system designed especially for operation within the higher frequency range known as the X-band of the microwave spectrum, and beyond.

In the transmission of microwave energy it is common practice to utilize cavity resonators as impedance matching devices or transformers. Such resonators consist in a known manner of a hollow space bounded by metallic walls and having a shape and dimension such as to cause the structure, upon excitation through suitable coupling means, to resonate at a desired frequency, whereby to set up a definite electric and magnetic field distribution pattern within the cavity being characteristic of the special type or construction of resonator being used. Coupling between the resonator and the associated input and output circuits or devices is effected -by means of inductive and/ or capacitative coupling elements, such as probe, loop, or slit couplings operably connecting the resonator with the microwave transmission system. Because of the relatively large surface area of the resonators, their transmission losses are relatively small, aside from the advantage of ruggedness of construction and ease of operation, well known to those skilled in the art.

The most-common form of cavity resonator utilizes a circular cylindrical resonating space, whereby the field distribution or pattern within the cavity is determined by the cylinder -functions of the structure. Ordinarily, the fundamental or so-called B01-mode of oscillation is e-mployed, in which case the electric field is vertical or at right angle to the end walls or cover plates of the cylinder and the magnetic field is circular and coaxial with the cylinder axis of the resonator. The wave length at the fundamental frequency is determined by the relation ?\=2.61R, wherein R is the radius of the resonator with air as medium filling the resonator space.

In order to vary the resonating frequency over a substantial operating range, suitable tuning means are employed preferably consisting of an adjustable tuning plug of conducting material and mounted in either of the end walls of the resonator, to variably project into the cavity.

Circuitwise, a cavity resonator of the referred to type represents an intermediate transformer in the manner of an ordinary -four-pole circuit, for the reason that the distributed basic constants, that is, the inductance and capacitance, behave like lumped values as far as the effect on the external circuits is concerned. The coupling elements advantageously consist of magnetic coupling loops for effecting connection with a coaxial transmission line, or of coupling slits or apertures for effecting connection with an input or output cavity.

A basic consideration in the design of the coupling elements or devices, in particular of a coupling loop, is the requirement that they exhibit a quasi-stationary behavior within the frequency operating range concerned. A coupling loop has the special advantage of enabling a simple and effective impedance matching and frequency band Patented Oct. 11, 1966 ICC width control of the resonator to a desired value by the t proper design or size as well as the orientation of the loop In order to enable a coupling loop to behave in quasistationary fashion, that is, to prevent the setting up of resonance phenomena along the loop wire, it is necessary that the electrical length of the wire remain at a value below M4 at the highest operating 4frequency for which the resonator is designed to operate. This requirement results in relatively small loop dimensions or areas as the frequency is increased, the area covered by the loop being reduced, for instance, to a little as a few mm.2 for wave lengths in the neighborhood of and above 6000 rnc. (megacycles), known as the X-band of the microwave frequency spectrum. These difiiculties assume an especially serious character where it is desired to embody a diode or the like conversion element directly in theresonator structure. For the latter purpose, it is customary to insert the diode in the lead to the resonator at a point immediately ahead of the coupling loop, in an effort to maintain the possibility of an adjustable coupling with the magnetic field set up within the cavity. Inasmuch as, within the higher microwave frequency range, both the loop and the diode assume the character of a trans-mission line, unforeseeable and uncontrollable frequency-dependent transformation effects may arise, involving the setting up of parasitic series and parallel resonances and, in turn, resulting in substantial transmission losses, reduced operating band width, and uncontrollable production and behavior of harmonics.

Accordingly, an important object of the present invention is the provision of a cavity resonator of the re-ferred to type embodying an improved coupling arrangement which is substantially devoid of the afore-mentioned and related drawbacks and defects inherent in the prior art arrangements and which is especially suited for operation in the higher range of the microwave Ifrequency spectrum, and beyond.

A more specific object of the invention is the provision of an improved coupling arrangement for cavity resonators designed for use in connection with operating frequencies within and beyond the upper range of the microwave frequency spectrum, which arrangement is both devoid of the drawbacks and defects of the previously known coupling devices and which will enable simple and effective impedance matching control within a predetermined operating frequency range and over a substantial range of working resistance variations.

Yet another object of the invention is the provision of an improved coupling arrangement for cavity resonators, especially for use in connection with operating frequencies in the higher microwave frequency range and beyond, which arrangement is especially suitable for direct incorporation therein of a diode or the like conversion element, such as a variable capacitance diode as used in harmonic frequency multipliers, parametric converters or amplifiers, or the like systems or devices.

The invention, both to the foregoing and ancillary objects, as Well as novel aspects thereof, will be better understood from the following detailed description, taken in conjunction with the accompanying drawings forming part of this specification and in which:

FIG. 1 is a longitudinal cross-section through a cavity resonator and improved coupling arrangement constructed in accordance with the principles of the invention;

FIG. 2 is a plan view of the resonator of FIG. 1, shown with the cover thereof removed;

FIG. 3 is a fragmentary view of FIG. l, showing an alternative output coupling of the resonator;

FIG. 4 illustrates a resonator according to the preceding figures embodying a variable capacitance diode, to act as a harmonic frequency multiplier; and

i, the like system.

Like reference numerals denote like parts in the different views of the drawings.

With the foregoing objects in view, the invention, according to one of its aspects, involves generally the provision of a main cavity being preferably, though not limitatively, of a cylindrical shape and bounded by a pair of opposed end walls connected by a side wall, with a nichelike lateral recess communicating with said main cavity and extending outwardly from said side wall, said recess having a dimension in the direction of the axis of said end walls, or in the direction of the electric eld set up within said cavity, being substantially less than the spacing distance between said end walls, or length of the electric iield lines wit-hin said cavity, to provide an lintermediate coupling space between said main cavity and the associated circuit element or device of a microwave transmission system in which the resonator is connected.

More specifically, coupling with said recess is effected through the inner conductor of a coaxial coupling element extending into the recess in a direction parallel to said axis and at a point relatively remote from the transition region between said cavity and said recess, while the degree of coupling is controlled by the provision of suitable first tuning means within said recess, consisting preferably of a tuning plug disposed at a point within or relatively close to the transition region between the recess and said cavity. By the further provision of said cavity with suitable second tuning means as well as additional `coupling means, microwave energy may be transmitted through said resonator within a predetermined operating Ifrequency range and at high etliciency or optimum impedance matching with the associate transmission circuits or devices, by the proper control and adjustment of the lirst and second tuning means, respectively, in a manner as will become further apparent as the description proceeds.

In other words, the invention utilizes an auxiliary cavity or space as a transition means to the main cavity, in such a manner as to overcome the ditiiculties involved in the use of a coupling loop according to the prior art arrangements, especially for operation in the higher frequency range of the microwave spectrum and beyond. An ancillary advantage of such arrangement is the possibility of a direct structural embodiment of insertion of a diode or the like solid state conversion element in the coaxial coupling element, to thereby eliminate the prior ditiiculties pointed out when using a loop as a coupling means, as will be further understood from the description of the drawings as follows.

Referring more particularly to FIG. 1, the numeral 1 denotes the cavity of a microwave resonator having a pair of end walls and a side wall and comprised, in the example shown, of a circular cylindrical casing 3 closed by a cover 2. Mounted in the bottom Iwall of and coaxially with the casing 3 is a rotatable, threaded tuning plug 4 the projection of which into said cavity may Abe varied in a known manner for the tuning of the resonator to a desired resonating frequency, said plug being electrically connected with the bottom of the casing through a number of flat or nger-like contact springs 4a. Further mounted in the side wall of the casing 3 is a magnetic coupling loop 5 which serves for the extraction of the microwave energy from the resonator cavity and feeding to a coaxial cable 6, in the example illustrated. In place of the loop 5, any other known coupling means may be employed, such as an aperture or slit coupling 13 feeding a load or output cavity 12, as shown in FIG. 3.

There is further provided in FIG. 1 a coaxial input coupling 7 having, Ifor instance, a low-ohmic impedance of say less than 50 ohms which it is desired to match with a given load or output impedance, of say 50 ohms, by the resonator acting as an impedance transformer. For this purpose, the cavity 1 is fitted, in accordance with the lpresent invention, with an auxiliary cavity in the form of a relatively flat niche or recess 8 being parallel to the end walls of the cylinder or main cavity and extending laterally outwardly from the casing 3, in the manner shown. The niche or recess 8 is traversed in the axial direction by the inner conductor 9 of the coaxial coupling element 7 being disposed at a point relatively remote from the transition region between the cavity 1 and recess 8. Besides, the recess 8 is fitted with a further tuning plug 10 disposed in or near said transition region. Advantageously, a further recess or depression 11 is provided in the bottom of the niche S and in line with the plug 10 for the extension of the adjusting or coupling range of the latter.

FIG. 2 is a plan view -of the resonator shown by FIG. 1 with 'the cover 2 removed. Essentially, the niche 8 may have the form of a semi-circular recess within the casing 3 and, similarly, recess 11 may be of semi-circular shape, as shown in the drawing. However, the shape of the niche is by no means critical and may be, for instance rectangular, if desirable for fabricating or other reasons. The width of the niche or recess 8 is advantageously about equal to the diameter of the cavity 1, or somewhat less, as 4s-h-own in FIG. 2. The spacing distance between the axes of the coupling conducto-r 9 and -of the tuning plug 10, FIG. l, is principally non-critical and determined mainly by practical and design considerations. For ordinary purposes, it has been found advantageous to utilize a spacing distance between elements 9 and 10 of the order of between 4 to M8, where )t represents the operating frequency in free space.

Besides, the height of the niche 8 may be varied within substantial limits without essentially affecting the transformation or impedance mat-ching characteristics of the device. However, the height of the niche should be substantially less than the height of the cavity 1, or spacing distance between the end walls of the cavity, its upper limit being about one half of the height of said cavity. In most cases, it is necessary to reduce the inductance of the inner conductor 9 traversing the niche or recess 8 to as low as possible a value, whereby to result in a relatively small height of the niche, such as in the order of from 1/3-1/s of the height of the main cavity 1.

There is thus provided by the two tuning plugs 4 and 10 mea-ns for the tuning land coupling control of the resonator, respectively, whereby to enable the matching of input and output impedance values diifering by a ratio as large as 20:1 and for operating frequencies varying withi-n a range of 15:1, while maintaining the transmission eiciency substantially constant throughout the operation.

The function of the niche 8 may be explained by the circular magnetic field produced by the current through the conductor 9 not only reaching into and faintly directly exciting the main cavity 1, but additionally exciting to a substantially higher degree the plug 10 acting as a series-resonant substitute network or circuit. As a consequence, the magnetic field set up by the plug 10 embraces the conductor 9, on the one hand, as well as the tuning plug 4, on the other hand, in such 'a manner lthat the ensuing interaction between the various fields results in a definite coupling between the conductor 9 and the cavity 1, determined by the -adjusting position, or natural resonance, of the plug 10 in relation to the operating frequency. The closer the resonant frequency of the plug 10 is to the operating or tuning frequency of the cavity 1, the closer will be the coupling, and vice versa. Such an arrangement exhibits, therefore, all the characteristics of a variable coupling device or transformer, substantially without involving the disadvantages or defects of a coupling loop at relatively high operating frequencies, as pointed out hereinbefore.

In use, adjustment may be effected by alternately displaci-ng the tuning plugs 4 .and 10 until the output energy of the resonator assumes a maximum, while the total operating bandwidth of the resonator is determined by the degree of the output coupling, that is, the size and orientation of the loop 5, in the example illustrated by FIG. 1. The resonating frequency proper of the resonator is practically not affected by the presencel of the niche 8, as long as the height of the latter remains less than about one half of the height of the cavity 1, in the manner pointed out.

In place of the circular cross-section of the cavity 1, as shown -in FIGS. 1 and 2, any other suitable shape `of the resonator may be used for the purposes of the invention, such for instance as a resonator having a rectangular shape or cross-section. Besides, a similar recess or niche coupling as shown may be used for the coupling of the resonator with its load or output circuit or device, that is, taking the pla'ce of the coupling loop 5 shown in FIG. 1.

,the form of Aa conventional P-N junction, are reversely biased by a suitable direct current source (not shown), whereby to exhibit a voltage-dependent capacitance effect within a substantial operating range. If excited by an alternating current voltage, as in the case of a harmonic generator or frequency multiplier as shown in FIG. 4, the fundamental frequency or applied input oscillation is dis- Y torted lto produce -a large number of harmonics one or more of which areextracted or segregated by the resonator, in

Vthe manner f-urther understood from the following.

l Again, in FIG. 4, the circular cylindrical cavity 1, consisting of the casing 3 and cover 2, is fitted with a coaxial line 6 as an output device. The coaxial input element 7 serves for applying the fundamental frequency to be multiplied, while a variable capacitance diode 14, in-

`serted in the inner conductor 1S of said element yand secured by a bush `116 in the cover 2, traverses the niche 8 in the axial direction of the resonator, in substantially l the same mannerY as in FIG, 1. The niche 8 is tted with a tuning or coupling plug 10 adjustable in the axial direction, or parallel to the conductor, also in the manner shown by FIG. 1. Y

The frequency multiplier or harmonic generator according to FIG. 4 operates by the fundamental frequency f being applied to the variable capacitance diode 14 via the coaxial line 7 and the capacitance formed by the inner conduetor 15 of said line and the casing 3. The dimension of the inner conductor andof the -air gap,` or capacitance, are chosen in such -a manner `as to cause the line 7 and the capacitance to form a transformer resonating at the frequency f, to result in the generation of a mixt-ure of harmonics of said fundamental frequency by the diode 14. A desired 'component of frequency nf, with n representing the multiplication factor, may be extracted by the proper design or adjustment of the tuning plug 4 of the cavity 1, in a manner understood from the foregoing.

FIG. 5 illustrates another example of a practical application of the invention as embodied in -a so-called parametric up-converter or frequency transformer. For the lat-ter purpose, the converter or frequency mixer comprises a pair of adjoining resonating cavities 1 and 1' composed of a common casing 3 and cover 2 and embodying a common variable capacitance diode 14. The cavity 1 is excited via the coupling loop 5 and coaxial line 6 by the microwave oscillation of relatively low frequency f1 to be converted, for which purpose 1 the cavity is tuned to the frequency f1 by means of the tuning plug 4. Output coupling is effected through the niche 8' and by the aid of the tuning or coupling plug 10', said niche being traversed by the variable capacitance diode 14, in the same manner as described hereinbefore. further microwave oscillation having an intermediate frequency f2, by which the input frequency f1 is to be increased is applied through the coaxial line 7, whereby the 1 diode 14, forming part of the inner conductor of the line, causes both oscillations to be intermixed in the manner customary in the operation of parametric converters or amplifiers. During this operation, the inner conductor of the line 7 and the capacitance formed thereby and the casing 3 act as parts of the transformer resonant to the frequency f2.

In the arrangement according to FIG. 5, the diode 14 simultaneously .traverses the niche or recess 5 of the cavity 1 which is -itted with a tuning plug 4. Both niches 8 and 8', in the exampleshown, communicate with one another, to form a common space between the main cavities 1 and 1' centrally traversed by the line 7. As a consequence, output energy at the sum frequency fyi-f2 may be extracted or derived from the cavity 1 by proper adjustment of the plugs 4 and 10 respectively, in a manner readily understood from the foregoing.

The composite cavity structure shown by FIGS. 4 and 5 may further be utilized as an input or output circuit or coupling for a multi-section bandpass filter or network.

According to a modified arrangement of the type shown by FIG. 5, the cavity 1 may be designed for a frequency f and cavity 1 may be designed for twice the frequency or 2f, while the intermediate circuit formed by the output conductor 7 may be short-circuited for high-frequency currents. If in such a case high-frequency energy of frequency 2f `is fed from a suitable pumping oscillator through the line 6 and if line 6 is connected with .a point of a wel-l-known circulator, the arrangement shown constitutes Ia so-called negative resistance parametric amplifier in respect Ito the input and output ends `of the circulator, in a manner well known in the art.

Furthermore, the multiple cavity arrangement according to FIG. 5 may be advantageously used in connection with a harmonic generator according to FIG. 4, by applying the fundamental frequency to the line 7 and deriving a pair of harmonics from the cavities 1 and 1' by the proper tuning and coupling adjustment, in the manner described. In place of the two cavities 1 Iand 1', additional cavities may be arranged radially around the `diode 14 disposed in a common recess, for the utilization of a corresponding number of harmonics derived from the respective cavities.

There is thus provided by the invention an arrangement enabling the effective coupling or ymatching of .a Working resistance varying within relatively wide limi-ts, in particular the loss resistance of a variable capacitance diode or the like conversion element, with a given load resistance designed for operation with frequencies in the upper region of the microwave spectrum and beyond,

whereby, despite the relatively large dimension of the diode or the like element in respect to the operating wave length, the diode may be directly or closely coupled with Ithe cavity resonator. Experiments have shown, for instance, that within a frequency range of from 6000-8000 mc. and for an impedance transformation from 2.5-50 ohms, the transformation losses may be kept below 1 db over .a ran-ge or bandwidth of 50 mc.

In the foregoing the invention has been described in reference to a few specific illustrative arrangements. It will be evident, however, that variations and modifications, as Well as the substitution of equivalent parts or elements for those shown herein for illustration, may be made Without departing from the broader purview and spirit of the invention as set forth in the appended claims. The specification and drawings are accordingly to be regarded in an illustrative rather than in a limiting sense.

I claim:

1. A cavity resonator comprising in combination:

(l) a resonant cavity bounded by a pair ofv opposed end walls connected by a side wall,

(2) a niche-like recess communicating with said cavity and extending laterally outwardly from said side Wall,

(3) said recess having a dimension in the direction of the axis of said end walls being substantially less than the spacing distance between said end walls,

(4) a coaxial coupling element having an outer conductor and an inner conductor, said inner conductor extending into said recess in a direction parallel to said axis and located at a point relatively remote from the transition region -between saidcavity and said recess, and

(5) adjustable tuning means located in said transition region.

2. A cavity resonator as claimed in claim 1, said tuning means being comprised of a tuning plug mounted in a wall of said recess and adjustable in a direction parallel to said axis.

3. A cavity resonator as claimed in claim 1, including tuning means for said cavity for adjusting 4the resonating frequency thereof.

4. A cavity resonator as claimed in claim 1, said recess having a lateral width substantially equal to the width of said cavity.

5. A cavity resonator as claimed in claim 1, the dimension of said recess in the direction of said axis being of the order of from 1/s-l/a of the distance between said end walls.

6. A cavity resonator as claimed in claim 1, said recess and said coaxial element forming a first coupling means for said cavity, and cooperating coupling means operably associated with said cavity.

7. A cavity resonator as claimed in claim 1, including a diode element serially structurally embodied in said inner conductor, to form an input for said cavity, further tuning means for said cavity to adjust the resonating frequency thereof, and output coupling means operably associated with said main cavity.

8. A cavity resonator as claimed in claim 1, including a variable capacitance diode serially structurally embodied in said inner conductor, to form an input for applying thereto a rst frequency, and further tuning means for said cavity for adjusting the resonating frequency thereof, to excite the same at a harmonic of said rst frequency.

9. A multiple cavity resonator comprising a pair of adjoining resonator units as defined by claim 1, each said liits including a cavity and a recess with the recesses of the units communicating with one another and traversed by a common central coaxial conductor, to provide a lirst coupling means for both said cavities, and individual tuning and second coupling means operably associated with each of said cavities.

10. A cavity resonator comprising in combination:

(1) a resonant cylindrical cavity having a pair of opposed end walls and a cylindrical side wall,

(2) a niche-like lateral recess communicating with said cavity and extending radially outwardly from said side wall,

(3) said recess having a width substantially equal to the diameter of said cavity and a height substantially less than the spacing distance between said end walls,

(4) a coaxial coupling element having an outer conductor and an inner conductor spaced therefrom, said inner conductor extending into said recess in a direction parallel to the cylinder axis and located at a point relatively remote from the transition region between said cavity and said recess, and

(5) adjustable tuning means located in the transition region between said cavity and said recess.

11. A cavity resonator as claimed in claim 9, said tuning means consisting of a tuning plug variably extending into said recess in a direction parallel to the cylinder axis.

12. A cavity resonator as claimed in claim 10, said recess having a depression in line with said plug, to extend the adjusting range thereof.

13. A cavity resonator as claimed in claim 9, including further tuning means to adjust the resonating frequency of said cavity, and further coupling means operably associated with said cavity.

14. A cavity resonator as claimed in claim 12, said further tuning means consisting of a coaxial adjustable tuning plug operably associated with said cavity.

15. A cavity resonator as claimed in claim 9, the axial height of said recess being of the order of from l-l/a the height of said cavity.

16. A cavity resonator comprising in combination:

(1) a cavity adapted to resonate at a'predetermined mode with its electric field lines having predetermined length and direction,

(2) a niche-like recess communicating with said cavity and extending therefrom in an voutward direction substantially normal to said eld lines,

(.3) said recess having a dimension in the direction of said lines being substantially less than the length of said lines,

(4) a coaxial conductor extending `into said recess in a direction parallel to said lines and located at a point relatively remote from the transition region between said cavity and said recess, and

(5) a tuning plug substantially parallel to said conductor and adjustably mounted in the wall of said recess, to variably project in-to said region.

References Cited by the Examiner UNITED STATES PATENTS 2,899,647 8/ 1959 Willwacher et al. 333-83 HERMAN KARL SAALBACH, Primary Examiner.

L. ALLAHUT, Assistant Examiner. 

1. A CAVITY RESONATOR COMPRISING IN COMBINATION: (1) A RESONANT CAVITY BOUNDED BY A PAIR OF OPPOSED END WALLS CONNECTED BY A WALL, (2) A NICHE-LIKE RECESS COMMUNICATION WITH SAID CAVITY AND EXTENDING LATERALLY OUTWARDLY FROM SAID SIDE WALL, (3) SAID RECESS HAVING A DIMENSION IN THE DIRECTION OF THE AXIS OF SAID END WALLS BEING SUBSTANTIALLY LESS THAN THE SPACING DISTANCE BETWEEN THE SAID END WALLS, (4) A COAXIAL COUPLING ELEMENT HAVING AN OUTER CONDUCTOR AND AN INNER CONDUCTOR, SAID INNER CONDUCTOR EXTENDING INTO SAID RECESS IN A DIRECTION PARALLEL TO SAID AXIS AND LOCATED AT A POINT RELATIVELY REMOTE FROM 